Time-lapse seismic is used to detect changes in a reservoir that occur between a first survey and a second survey after a predetermined time. The time between two successive surveys is so determined that a change in the underground formation can be detected. In many instances time-lapse seismic surveying is done to study changes in an underground reservoir.
An example of time-lapse seismic monitoring is described in U.S. Pat. No. 4,969,130. This publication discloses monitoring the fluid contents of a petroleum reservoir, wherein a reservoir model is employed to predict the fluid flow in the reservoir, which monitoring includes a check on the reservoir model by comparison of synthetic seismic surveys with the observed seismic surveys. If the synthetic output predicted by the model agrees with the observed seismic data, it is assumed that the reservoir has been properly modelled. If not then the reservoir model, in particular its reservoir description, is updated until it predicts the observed seismic response. The seismic survey and the technique used to update the reservoir model may be periodically repeated during the productive life of the reservoir, so as to ensure that the revised reservoir description predicts the observed changes in the seismic data and hence reflects the current status of fluid saturations.
In the known method a synthetic seismic survey (or a synthetic seismogram) is produced in order to allow a comparison with the recorded seismic surveys. The synthetic seismic survey is made on the basis of a reservoir model that allows predicting the flow of fluids through the reservoir, and a geomechanical model that allows predicting the deformations of layers in the underground formation.
An essential property for modelling the underground formation is the dependency of the seismic velocity on the state of stress in the formation. The seismic velocity is the propagation rate of a seismic wave through the underground formation, and the state of stress is defined by the magnitude and direction of normal and shearing stresses acting on an element of the underground formation, which element has a known orientation. The state of stress in the underground formation is represented by a stress tensor that is characterized by its stress components. An example of a stress component is the vertical stress in the underground formation. The components of the stress tensor can be combined in many ways to yield a stress condition. A first example of the stress condition is the vertical stress. Another example of the stress condition is the average of the normal stresses, and a further example is a principal stress. The stress conditions can be used as a measure for the state of stress.
The seismic velocity that is of interest here is the velocity determined by the travel time from a seismic source, along a path through the underground formation, to a seismic receiver that is arranged at a distance from the seismic source. The distance between source and receiver is preferably at least 100 m, typically several hundreds of meters to kilometers. In this way, seismic velocities and travel times along pathways in the same order of magnitude are probed as in seismic surveying. This is different from sonic logging techniques such as disclosed U.S. Pat. No. 5,197,038, in which sonic velocity locally at a distance of the order of 1 meter is probed. Deriving long-distance seismic velocities from sonic measurements is a very complicated task.
It is an object of the present invention to provide a simple method of determining the relation between the seismic velocity and the state of stress in the formation that can be carried out in-situ. More in particular the present invention relates to determining a relation between a change in the seismic velocity and a change in the stress for an underground formation that is located under a surface having time-changing surface loading conditions.